This invention relates to flue gas cleanup and, more particularly, to removing nitrogen oxides and sulfur oxides from a combustion effluents emitted during the direction combustion of coal.
Flue gases emitted during the direct combustion of coal often contain undesirable levels of sulfur oxides (SOx) and nitrogen oxides (NOx) which, if untreated, might pollute the atmosphere. Sulfur oxides in the presence of water can form sulfuric acid causing acid rain. Nitrogen oxides may cause smog by photochemical reaction with hydrocarbons in the atmosphere.
Over the years, various methods have been suggested for controlling and/or removing sulfur oxide and/or nitrogen oxide emissions. Methods for removal of nitrogen oxides include low excess air, staged combustion, reburning, ammonia injection, and selective catalytic combustion. The typical percent removal of nitrogen oxides by these methods range from 30% to 90%.
Control techniques for nitrogen oxides can be separated into those which utilize combustion modifications to convert the nitrogen oxides to nitrogen and those which inject ammonia either into the furnace or into a cataytic reactor to reduce the nitrogen oxides concentration. Although combustion modifications are typically quite inexpensive, they are usually limited in the amount of reduction which can be achieved to about 50%. While injection of ammonia into the flue gas stream can be effective, very careful control of the injection temperature is required to achieve a high conversion efficiency. In an industrial or utility situation, this control can be difficult to maintain.
Generally, the removal of sulfur oxides can be accomplished with the injection of dry calcium or the use of varous complex expensive equipment installed downstream of the boiler. Calcium can also be obtained from calcium carbonates or calcium hydroxides and can be injected directly into the combustion chamber to absorb sulfur oxides. However, this is relatively inefficient, requiring large amounts of calcium. In catalytic cracking units, sulfur oxide control processes usually occur in the regenerator. In one widely used process, sulfur oxides are captured in the regenerator with sulfur oxide-capturing acceptors and subsequently converted to hydrogen sulfide in the reactor and the hydrogen sulfide is withdrawn with the product stream from the reactor and treated in a sulfur recovery plant.
Flue gas streams discharged from regenerators, power plants or other combustors are commonly directed through one or more dedusters, such as flue gas scrubbers, electrostatic precipitators, cyclones bag houses, granular bed filters, or other filters, in order to remove particulates from the glue gas stream.
Typifying prior art methods for controlling sulfur oxide and/or nitrogen oxide emissions, or other pollutants, prior art uses of sodium chloride, iron oxides, or manganese-containing compounds, as well as other prior art processes, equipment, and compositions, are those described in U.S. Pat. Nos. 621,884; 1,150,839; 1,555,590; 1,895,724; 2,014,686; 2,083,894; 2,493,218; 2,493,911; 2,522,426; 2,575,520; 2,863,824; 2,992,895; 3,023,836; 3,068,627; 3,251,649; 3,264,801; 3,320,906; 3,369,504; 3,389,829; 3,501,897; 3,723,598; 3,755,535; 3,760,565; 3,778,501; 3,823,676; 3,832,445; 3,835,031; 3,840,643; 3,846,536; 3,892,677; 3,987,146; 4,001,376; 4,006,066; 4,014,981; 4,039,478; 4,115,518; 4,153,534; 4,153,535; 4,164,545; 4,181,705; 4,206,039; 4,218,344; 4,221,677; 4,233,175; 4,233,276; 4,238,317; 4,241,033; 4,254,616; 4,258,020; 4,267,072; 4,300,997; 4,323,542; 4,325,811; 4,369,109; 4,369,130; 4,376,103; 4,381,991; 4,405,443; 4,423,019; 4,443,419; 4,500,501; 4,540,554; and 4,552,734.
These prior art methods, equipment, and compositions have met with varying degress of success.
Another by-product of burning coal in particular is coal ash. The ash is a mixture of several phases of inorganic residues that accompany the coal. Because of its complexity and its insolubility, the toxicological, chemical, and morphological properties of the ash are not easily evaluated and new product development using coal ash has consequently been inhibited. Consequently, plants that process coal are also accumulating huge ash piles which becomes sources of both soil and water pollution. Significant mantenance expenses are incurred even though the ash is a potential source of valuable minerals.
It is, therefore, desirable to provide an improved process for cleanly burning coal and minimizing pollution, which overcomes most, if not all, of the preceding problems.